Flow Measurement Apparatus and Method

ABSTRACT

The velocity of fluids containing particles that scatter ultrasound can be measured by determining the Doppler shift of the ultrasound scattered by the particles in the fluid. Measuring fluid flow in cylindrical vessels such as blood vessels is an important use of Doppler ultrasound. This invention teaches using various configurations of cylindrical diffraction-grating transducers and cylindrical non-diffraction-grating transducers that suppress the Doppler shift from non-axial components of fluid velocity while being sensitive to the Doppler shift produced by axial velocity components. These configurations thus provide accurate measurement of the net flow down the vessel, even when the fluid flow is curved or not parallel to the vessel wall.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/437,945, filed on Jan. 31, 2011. The disclosure of the aboveapplication is incorporated herein by reference in its entirety for anypurpose.

REFERENCE TO GOVERNMENT FUNDING

This application was made with partial Government support under contract2R44HL071359 awarded by the NHLBI of the National Institute of Health.The Government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention generally relates to flow measurement and moreparticularly to measurement of velocity of fluid containing ultrasoundscatterers flowing through a pipe or blood vessel using Dopplertechniques.

BACKGROUND OF THE INVENTION

As the function of a pipe is to carry a volume of fluid from one pointto another, the flow volume passing through it determines how well it isfunctioning. We will refer to fluid-carrying pipes as “vessels”, as inan exemplary application of measurement of flow in blood vessels.However, the teaching of how to measure flow volume in the presentinvention is applicable to measurement of flow of any liquid thatcontains particles that scatter ultrasound, e.g. milk, slurries, watercontaining bubbles, etc, as well as blood. Using ultrasound Dopplertechniques to measure the flow of blood is well-known. Red blood cellsact as scatterers of ultrasound in the MHz frequency region, and whenthey are insonated by a beam of ultrasound their movement creates aDoppler shift in the scattered sound. The amount of shift in frequency,also known as the Doppler shift, is proportional to the number ofwavelengths of ultrasound per second that the red blood cell moves. Thisproportionality is the cosine of the angle between the velocity of thescatterer and the direction of propagation of the ultrasound beam. Asthe peak velocity of blood in human blood vessels is about 1meter/second, using ultrasound in the low MHz, where the wavelength is afraction of a millimeter, leads to Doppler shifts in the low KHz, i.e.in the audible region, in which detected signals can be heard. Bydetecting the Doppler shifts, the velocity of the blood cells can becalculated. See, for example “Doppler Ultrasound” by Evans and McDicken,2^(nd) Ed, J. Wiley and Sons, New York 2000, for a thorough discussionof the use of Doppler ultrasound in measuring blood velocity.

Doppler velocity measurements are usually made with a combination of animage of the vessel with a graphic presentation of the Doppler shift vstime, known as “duplex Doppler”. The translation of the measured Dopplershift to the more useful velocity generally assumes the flow to beparallel to the axis of the vessel. Other techniques that have beenproposed require multiple frequencies or complex mathematicalmanipulations of the signal. Conventional ultrasound methods oftensample only a small portion of the flow through a vessel and extrapolatea flow from that small sample. This, however, frequently causesmeasurement to be inaccurate.

The present invention provides an apparatus and method to overcome thesedrawbacks in the existing Doppler measurement art. In the presentinvention, we teach a new configuration for direct application to thevessel to allow accurate measurement of flow carried by the vessel.Because a rotational symmetric cylindrical transducer is used, the flowis not required to be parallel to the axis of the vessel in order to bemeasured accurately. Moreover, unlike conventional ultrasound methodsthat sample only a small portion of the flow through a vessel andextrapolate a flow volume from that small sample, the present inventionmeasures flow through most of the cross-section of the lumen, thus canprovide an accurate measurement.

SUMMARY OF THE INVENTION

The present invention uses a new configuration of transducers, acombination of a cylindrical DGT (or diffraction-grating transducer) anda cylindrical non-diffraction-grating transducer (or non-DGT). As willbe seen, these cylindrical transducers provide special characteristicsthat produce the improved operation over non-cylindrical transducers.FIG. 1 shows the configuration in side view, i.e. a slice through thetube 101 along its axis. The wavefronts launched by the cylindrical DGT102 on opposite walls of the vessel produces planes of “standing waves”,i.e. equiphase plane as expressed by lines 104, perpendicular to thevessel axis. A moving scattering particle, e.g. a red blood cell, withcomponent of velocity along the axis, scatters a signal of changingphase as it crosses the different equiphase planes; the scatteredultrasound is received by the cylindrical transducer 103. The scatteredsignal has a changing phase with respect to the driving signal on theDGT, i.e. a Doppler shift in the received signal compared to thetransmitting frequency. Any velocity component perpendicular to the axisof the vessel is parallel to the equiphase planes 104, and will scatteronly a constant-phase replica of the transmitted signal, producing asignal with zero Doppler shift from the cylindrical transducer.

Both DGT and non-DGT transducer are rotationally symmetric, so in acomplete vessel equiphase planes are formed, rather than equiphaselines. Therefore in those regions Doppler shifts arise only from theaxial component of velocity and do not arise from any non-axialcomponents.

As it is only the axial component of velocity that produces flow througha vessel, the Doppler signal produced by this configuration accuratelymeasures the flow down the vessel, and is not affected by non-axialflow.

According to one aspect of the present invention, a configuration of acylindrical DGT next to a cylindrical non-DGT is used so that onlyvelocity that is along the direction of the cylinder's axis, which isproportional to the flow direction 110 (in FIG. 1), generates a Dopplersignal. This is the unique contribution of using cylindrical DGT with acylindrical non-DGT.

With reference to FIG. 8, in a typical curved human blood vessel, thedirection of blood flow (1010) is not parallel to the axis of the vessel(1020). Because the vessel is a cylindrically symmetric structure, theparticle velocity can be resolved into axial and perpendicular vectorcomponents, and it is only the velocity along the axis that contributesto volume flow. Therefore, the present invention is advantageous overexisting Doppler ultrasound means of measuring flow such that it canprovide accurate measurement of flow when the velocity is not parallelto the vessel wall.

The Doppler frequency generated by a scatterer is proportional to itsvelocity, and different scatterers may move at different velocities,mostly with scatterers near the center of the vessel moving at highervelocity than the scatterers near the wall. It is a well-knowntechnique, as taught by Evans and McDicken in Chapter 12, VolumetricBlood Flow Measurement, in “Doppler Ultrasound”, 2^(nd) Ed, J. Wiley andSons, New York 2000, that the total flow can be calculated from thespectrum of the Doppler signal by recognizing that the amount of powerat each frequency bin of the Doppler spectrum represents the number ofscatterers moving at the corresponding velocity; summing the Dopplerpower corresponding to each velocity therefore gives the total flowvolume. The configuration according to the present invention insonatesalmost the entire lumen so all parts of the vessel's flow contribute tothe measurement of flow.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 shows the cross-section of the basic configuration, thecombination of a cylindrical DGT with a cylindrical non-DGT transduceraccording to one aspect of the present invention.

FIG. 2 shows the wavefronts arising from the interaction of the waveslaunched from opposite sides of the vessel by the cylindrical DGTaccording to one aspect of the present invention.

FIG. 3 shows one embodiment according to one aspect of the presentinvention.

FIG. 4 shows an alternative embodiment utilizing a double-beam DGT thatcould be advantageous for certain conditions.

FIG. 5 shows one embodiment according to another aspect of the presentinvention.

FIG. 6 shows the differing propagation paths to be considered inmeasuring the flow of attenuating media according to another aspect ofthe present invention.

FIG. 7 shows one embodiment for the use with attenuating fluidsaccording to another aspect of the present invention.

FIG. 8 shows how curved blood vessels produce blood velocity vectorsnon-parallel to the vessel axis.

FIG. 9 shows the forming of a cylindrical DGT by rolling up adouble-beam DGT along an axis according to one aspect of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, cylindrical DGT 102 establishes equiphaseplanes 104. The DGT 102 produces beams at an angle of θ, with respect tothe perpendicular to the vessel axis 110, where sin θ=λ/d, where λ isthe wavelength of the ultrasound in blood at the frequency used, and dis the periodicity, i.e. the distance between points of equal phase, ofthe grating of the DGT. The beams from the opposite walls of the vesselwill cross, producing a standing wave pattern. For example, thewavefront coming from the top DGT 121 will intersect with the wavefrontlaunched at the same time (i.e. same phase) at the corresponding pointat the bottom of the vessel 122, and those wavefronts must be in phasewhere they intersect, producing an equiphase front. Similarly, the nextcorresponding points 123 and 124 on the DGT will intersect at the nextequiphase front, etc.

With reference to FIG. 2( a), in a cylindrical DGT 200, if we take acentral electrode of the transducer at 220, the propagating path 205from the top is the same as propagating path 204, from the bottom of thetransducer. Where they intersect, at 201 (at equal distance C from thetop or bottom wall of the vessel), on the plane 210, they have the samephase, as they were launched from the same element and traveled the samedistance to reach 201.

If we now consider a new point on the plane 210 a distance x away, at202, we see that there is a path 207 from 221 on the upper part of thetransducer that intersects at point 202 with a path 206 from point 222.

The lengths of the paths 207 and 206 are different. However, we can showthat the change in phase in the two paths 206 and 207 when moving fromthe equal-length intersection point 201 to the arbitrary point 202cancel, i.e. the increase in path length of one path is exactly the sameas the decrease in path length of the other, so the net change in phaseof the intersection point 202 is zero. As the distance x is arbitrary,this shows all points on that plane 210 have the same phase, i.e. it isan equiphase plane. This can be illustrated as below.

The propagating paths are always at an angle of θ to the perpendicular,determined by the spacing of the electrode elements. With reference toFIG. 2( a), we can see that tan θ=D1/(C−x)=D2/(C+x) where D1 is distancefrom plane 210 to the point where propagating path 207 begins, D2 isdistance from plane 210 to the point where propagating path 206 begins,and D0 is the distance from plane 210 to the point where propagatingpath 205 and 204 begin. We can state

D1=(C−x)tan θ and D2=(C+x)tan θ. D0=C tan θ, so

D2−D0=x tan θ and D0−D1=x tan θ

i.e. the originating point D1 along DGT 200 for propagating path 207 isthe same distance from D0 toward the right as the originating point D2for propagating path 206 is from the left and away the right for theother, are equal. As the phase along a DGT is continuously increasing,every d increasing by 2π, that means the change in phase for one path isincreasing by the same amount as the other is decreasing; therefore thenet change in phase for the sum of the two paths 206 and 207 will alwaysremain the same as the phase at point C, for arbitrary distance x or forany angle θ. Hence, the plane 210 is an equiphase plane. Hence, ascattering particle moving along these planes will always scatter thesame phase, i.e. will not produce a Doppler frequency shift. A particlemoving perpendicular to these planes will produce a change in the phaseof the scattered signal, a Doppler frequency. Hence, velocity along theaxis will produce a Doppler shift, velocity perpendicular to the axiswill produce none, as desired for measuring flow through the vessel.

The amount of change in phase with velocity can be determined bydetermining the change in phase between equiphase planes. With referenceto FIG. 2( b), a new set of propagating paths to a new central point 231is shown. By the characteristic of parallel lines that the distance atthe same angle between two parallel lines must be the same, advancing adistance d, i.e. the periodicity of the DGT from 220 to 232, mustadvance the equiphase planes by the same distance, d, as shown.

The number of planes per unit of time Δt that a particle moving alongthe axis crosses is expressed by Np=D/d, where D is the distance theparticle travels, and d is the distance between these equiphase fronts,also the periodicity of the DGT. Because “D”=vΔt, Np=D/d=vΔt/d. TheDoppler shift f_(D) is expressed as the change of phase with time, whichis the definition of radian frequency. Therefore,f_(p)=Δθ/Δt=ω=2πNp/Δt=v/d.

Embodiment 1

While there are various methods of constructing ultrasonic transducers,in one exemplary method, DGT and non-DGT are constructed using apiezoelectric plastic transducer material, such as P(VDF-TrFE) coated onan electroded flexible plastic substrate such as polyimide film of 1-milthickness. The electrodes for a DGT are placed such that their phasewill advance 27 every d for the desired beam angle θphotolithographically applied. In this exemplary embodiment, withreference to FIG. 3, a beam can be produced by the DGT at an angle ofsin θ=λ/d. As is taught in U.S. Pat. Nos. 5,488,953 and 5,540,230, bothto Vilkomerson, when four elements driven in phase advancement of π/2make up one period separated by d/4, a single beam at this angle isproduced. When 2 elements, separated by d/2 and driven at 180° are used,two symmetrical beams are produced. These patents are incorporated byreference.

The film transducer can be wrapped outside or inside of a vessel orpipe. An exemplary structure, as shown in FIG. 9, is first fabricated ona flat film and then wrapped around a vessel. With reference to FIG. 3,a film transducer that is wrapped inside a vessel is shown. Thoseskilled in the art recognize that signal cables (not shown) will berequired to drive the DGT, and to carry the received signal from thenon-DGT cylindrical transducer. The cables can be coax, stripline orother types readily available on the market.

According to one aspect of the present invention, the plastic substrateis spin-coated with a piezoplastic layer such as 15 micron layer ofP(VDF-TrFE) which in turn is sputter-coated with a conductor layer andthe piezoplastic poled. The conductor layer is also used for producingthe electric field needed for transducer operation. Construction of suchflexible film DGT's were described in “A Flexible Implantable Sensor forPostoperative Monitoring of Blood Flow” by Cannata et al presented atthe Annual Meeting of the American Institute of Ultrasound in Medicine,San Diego, Mar. 28, 2010.

Once the Doppler signal is obtained, it is processed in the usual mannerof continuous wave (known as CW, in distinction from PW, pulsed waveDoppler that utilizes pulses of insonating ultrasound) Doppler e.g.Chapter 6. Signal Detection and Preprocessing, in “Doppler Ultrasound”by Evans and McDicken, 2^(nd) Ed, J. Wiley and Sons, New York 2000 todetermine the velocity components and the flow.

If the velocity of the red blood cell is 1000 mm/sec (a velocity ofblood found in the body) and d, the periodicity of the DGT, is 100microns, the Doppler shift will be 1000/0.1=10 kHz. Similarly, givenperiodicity of the DGT and measurement of Doppler shift, the velocitycan be determined from the relationship above, v=d*f_(D).

Assuming a 6 mm diameter blood vessel is to be measured, for example, weuse a 30 MHz ultrasound frequency. This frequency is chosen to produce astrong Doppler signal, as the amount of scattering of ultrasoundincreases by a factor of the frequency to the fourth power, while makingthe dimensions of the diffracting grating not so small as it isdifficult to fabricate. The ultrasound wavelength at this frequency inblood is ˜50 microns. Using a 45° beam, as shown in FIG. 3, and usingsin θ=λ/d, λ/d=0.707, so the periodicity d is set as 50/0.707=71microns. The total center-to-center spacing (for four-elementconfiguration) is therefore (71)/4=17.75. To enable poling thetransducer elements, there must be sufficient space between theelectrodes, but to provide enough active area, the electrode should notbe too thin. Generally, roughly equal width to separation between theelectrodes is used. According to one aspect of the present invention,the electrodes are made 10 microns wide for simplicity in fabrication,and the space between the electrodes becomes 7.75 microns wide. Withreference to FIG. 3, the length of the DGT 304 can be designed to be thesame as the diameter, 6 mm, so the 45° beam from one edge of the DGTreaches the bottom of the vessel at the base of the non-DGT cylindricaltransducer.

Note, as shown all of the cross-section of the lumen is insonated, andfor the 0.75 mm long transducer, most of the lumen within thistransducer, indicated by shading 305 in FIG. 3( a) is filled by thecrossed beams from the top and bottom parts of the DGT. The unshadedportion of the lumen 306, while insonated by ultrasound, is not filledwith the standing wave pattern, thus it will not discriminate againstnon-axial components of velocity as in the rest of the lumen. In caseswhere walls keep the flow lines parallel to themselves, non-axial flowwill not exist. However, no doubt some non-axial flow will be detectedin highly turbulent flows. Changing the beam angle and the dimensions ofthe DGT and non-DGT cylindrical transducers allows filling more or lessof the lumen. For example, as shown in FIG. 3( b) where increased beamangle, increased DGT dimension 314, and reduced dimensions of thenon-DGT 313, fills more of the lumen with wavefronts. If higher degreesof non-axial flow are expected, more of the lumen should be filled, andvice versa, so the optimum design for a particular situation must fitthe measurement task.

Embodiment 2

Another embodiment according to the present invention uses a double-beamDGT, as shown in FIG. 4. Two non-DGT cylindrical transducers, 402 and403, are placed at either end of the double-beam DGT 404. Double-beamDGT has been taught in U.S. Pat. No. 5,540,230 issued to Vilkomersonand, because it requires only two elements per period d rather than 4elements for the single beam DGT, it is easier to fabricate and connectto.

With reference to FIG. 4, at each end standing wave patterns will be setup, and Doppler signals will result from the axial components but notfrom the non-axial ones. The Doppler signals would be analyzed forvelocity and flow by conventional CW Doppler processing means as in theprevious embodiment, recognizing that the frequencies from thecylindrical transducer at one end 405 will be opposite in phase-shiftdirection than from that on the other end 406, i.e. a particle goingleft to right will produce a positive Doppler shift in 402 (as the phaseis decreasing) and a negative Doppler shift in 403, as the phase isincreasing. Using a single non-DGT at one end of the double-beam DGTwould also be functional, but half of the acoustic energy would not beutilized for Doppler measurement in this case.

Embodiment 3

With reference to FIG. 5, a single non-DGT 502 is placed between twosingle-beam DGT's 503 and 504 that are driven so that both beams crossunder the non-DGT 502. In this way, a minimal amount of lumen is leftunfilled by crossed-beam standing waves 505 and 506, each contributed bycorresponding DGT's 503 and 504, respectively. This may be important formeasuring very turbulent flow with non-axial components of flow evennear the wall.

It should be noted that in the case of FIG. 5 one DGT must be driven ata different frequency than the other; otherwise, the direction ofaxial-moving particles interacting with one DGT's beam will not be ableto be differentiated from the other direction. Another way ofconsidering this is to recognize that this system is now symmetricalaround the non-DGT, so the direction of flow cannot be determined.

Driving a DGT at different frequencies has been taught in U.S. Pat. Nos.5,488,953 and 5,540,230, both to Vilkomerson. By driving one DGT at adifferent frequency, the Doppler shift from each direction isdifferentiated from the other by heterodyning the received signal withthe appropriate driving frequencies. The Doppler shift, recovered afterthis heterodyning, is previously shown as v/d in magnitude; however, asthe direction of flow is toward one DGT and away from the other, theDoppler shifts are of opposite sign.

Attenuating Media

The equiphase fronts used in the present invention are produced by equalstrength beams intersecting. In attenuating media, however, theinteresting beams are no longer of equal strength, but have beenattenuated in their propagation through the medium. For example, in FIG.6, which shows the embodiment of FIG. 3, the propagation paths 603 fromthe center of the DGT (top and bottom), where the Doppler signalsoriginate, to the equiphase plane at the front of the region, are equal.So in an attenuating medium they will be of equal intensity. However,the propagation distance along the path 602 is much longer than thedistance along the path 601, while both paths intersect at the equiphaseplane. In an attenuating medium, therefore, the beams will not be ofequal power as is desired to form the standing wave equiphase front. Itcan be seen that, other than the central point reached by propagatingpaths 603, the beams from the top and bottom of the DGT will not be ofequal strength due to greater path length through the attenuatingmedium. In particular, at the high frequencies required for buildingsmall and easily implanted devices, attenuation of ultrasound in bloodcan be significant, e.g. 11 dB/cm at 30 MHz. (Treeby B E, Zhang E Z,Thomas A, Cox B, Measurement of the Ultrasound Attenuation andDispersion in Whole Human Blood and Its Components from 0-70 MHz,Ultrasound in Medicine and Biology 2011; 37:289-300.)

While it is possible to calculate the propagating path lengths fromdifferent points on the DGT, the differences in beam strength caused bythe medium's attenuation can be compensated for by exciting the far“edge” of the DGT more strongly than the “near” edge. For example, inFIG. 6, the left edge of the DGT producing the 602 beam should beexcited by a higher voltage than the right edge of the DGT 610 thatproduces the beam 601. By the symmetry of the device, driving the faredge more strongly will also equalize the beams 605 and 606 thatintersect at the top of the equiphase front.

In an exemplary system, Embodiment 1 was of a 30 MHz DGT measuring a 6mm diameter vessel. The DGT was 6 mm long, and produced a beam at 45degrees. Referring to FIG. 3, because the cosine of 45° is 1/√2, thebeam from the far edge intersects with the beam from the near edge afterpropagating through 6×√2=8.5 mm of blood. Using the figure of 11 dB/cmattenuation at 30 MHz, and using the 8.5 mm of extra travel through theblood, we can calculate that we need to drive the far edge of the DGTwith voltage 2.8 times greater than the far edge. Note that the beamfrom the near edge intersects with the beam from the far edge, sodriving the element at the far edge 2.8 times more strongly than theelement at the near edge will result in balanced beam strength at theirintersection.

Note that the beam 603 from the center of the top-portion of the DGTintersects with the beam 603 from the center of the bottom DGT, withboth beam portions going the same distance from the DGT to the point oftheir intersection. These beams have gone exactly half as far as thebeam from the far edge of the DGT, or 3×√2=4.24 mm, leading to 4.67 dB,or 1.72 more attenuation than a beam from the near edge of the DGT.Similar calculations can be done for each element in the DGT. Using anexemplary resistive “ladder” as shown in FIG. 7, each element can bedriven at the correct voltage to compensate for the attenuation fromeach element to the region of Doppler signal generation 620. Because theattenuation is logarithmically related to the propagation distance, thevoltage drives are not perfectly linearly related to position but ratherlogarithmically related.

In another exemplary system described in Embodiment 2, using the 6 mmlength DGT and double-beam at 30 MHz and 45° would require 85 pairs ofelements, 15 microns wide and separated by 20.5 microns driven. Bysimulating the circuit, i.e. making a circuit equivalent with thecapacitance of each element attached to the nodes of a resistor-inseries-with-an-inductor ladder arrangement, with the other end atground, the proper values for resistance of the interconnects connectingthe nodes of the ladder can be calculated to achieve the desiredvariation in voltage from the far edge to the near edge.

According to one aspect of the present invention, we can form a goodapproximation to the needed excitations for the described DGT bycontrolling the width, which determines the resistance, of theinterconnects between the elements of the DGT considered. With referenceto FIG. 7, calculation shows that 0.316 ohms resister (610) between thefirst 42 elements, and 0.516 ohms resister (620) between the final 43elements would be needed. This would correspond to gold lines 600 nmthick 5 microns wide for the first part of the array (630) and 7.25microns wide for the second part of the array (640). An element 15.5 mmlong and 30 microns wide (650) produces the value of 19 ohms needed tocomplete the ladder network; the total impedance of this DGT is 50.6ohms, an appropriate value for driving with standard coax cableconnections. With this configuration, it can be shown that the error inbalance of the beams across the lumen is less than 2%. As excessivephase shift would also cause irregularities in the beam pattern. Thetotal phase shift error due to the inductances in this array was lessthan ±5°.

This general approach described for compensating for the attenuation inattenuating media is only exemplary. Other methods of changing the drivevoltage to compensate for attenuation are also possible, as known tothose skilled in the art.

These examples are not meant to be exhaustive but rather to indicate thedifferent ways those skilled in the art will be able to utilize theprinciple of establishing planes of standing waves perpendicular to theaxis to detect only axial motion, and therefore to make accuratemeasurement of the net flow through the vessel.

Variations of the above disclosed embodiments can also be made toaccomplish the same functions. For example, with regard to Embodiment 2,a single non-DGT at one end of the double-beam DGT may be used, but halfof the acoustic energy would not be utilized for Doppler measure.

Further, the non-DGT transducers used to receive the scatteredultrasound in the various embodiments can be replaced by DGT's if thespacing d′ of these DGT's is different from that of the d of theinsonating DGT's. As discussed in relation to FIG. 2( a), the equiphaseplanes of a DGT with spacing d′ will be d′; a particle moving along theaxis, then, would produce a Doppler shift of V/d from its motion throughthe insonating planes produced by a DGT of periodicity d and a furtherDoppler shift of V/d′ by passing through the receiving planes of areceiving DGT with periodicity d′.

For example, if a DGT was in place of the non-DGT in FIG. 6, then theDoppler shift for a particle going left to right down the axis of thevessel would be −(V/d+V/d′), producing a negative Doppler shift as theparticle is moving away from both transducers.

Still further, if two DGT's replaced the two non-DGT's as receivers inFIG. 4, both would produce Doppler shift signals of (V/d−V/d′), onepositive and one negative as both receivers would be producing phaseplanes of opposite phase progression.

Still further, the flexible film used as the substrate for the DGT andnon-DGT structures can also be other plastics such as PET, polyethyleneetc. Still further, the piezoplastic transducer material can beconstructed from Nylon 7, Nylon 11 or other such piezoplastic as well asP(VDF-TrFE); and this piezoplastic layer can be spin-coated, dipped,brushed on, or in other ways formed as a layer on the film substrate.

Still further variations, including combinations and/or alternativeimplementations, of the embodiments described herein can be readilyobtained by one skilled in the art without burdensome and/or undueexperimentation. Such variations are not to be regarded as a departurefrom the spirit and scope of the invention.

1. An ultrasonic transducer apparatus for measuring a velocity of afluid flowing through a lumen comprising: a cylindricaldiffraction-grating transducer coupled with a predetermined number ofelectrode elements for exciting an ultrasound beam at a predeterminedangle with respect to the lumen axis; a cylindricalnon-diffraction-grating transducer coupled with an electrode element forreceiving a scattered signal having a changing phase with respect to theultrasound beam excited by the cylindrical diffraction-gratingtransducer; whereby said cylindrical diffraction-grating transducer andsaid cylindrical non-diffraction-grating transducer are adjacent to eachother and are rotational symmetric about the center of the lumen.
 2. Theultrasonic transducer apparatus of claim 1, further comprising a voltagecorrecting circuit operative to compensate for attenuating fluid.
 3. Theultrasonic transducer apparatus of claim 1, wherein said cylindricaldiffraction-grating transducer and said cylindricalnon-diffraction-grating transducer are wrapped inside or outside of thelumen.
 4. The ultrasonic transducer apparatus of claim 1, wherein saidcylindrical diffraction-grating transducer and said cylindricalnon-diffraction-grating transducer comprising a flexible electrodedplastic film, a piezoplastic transducer layer, and a conductor layer. 5.The ultrasonic transducer apparatus of claim 4, wherein said plasticfilm is constructed from one of a polyimide and PET material.
 6. Theultrasonic transducer apparatus of claim 4, wherein said transducerlayer is constructed from one of a P(VDF-TrFE), Nylon 7, and Nylon 11material.
 7. The ultrasonic transducer apparatus of claim 1, wherein theultrasound beam is operative in the MHz frequency range.
 8. A method ofmeasuring a velocity of a fluid flowing through a lumen based on aDoppler technique comprising the steps of: coupling a cylindricaldiffraction-grating transducer and a cylindrical non-diffraction-gratingtransducer such that they are adjacent to each other; exciting anultrasound beam at a predetermined angle with respect to the lumen axisfrom said cylindrical diffraction-grating transducer; receiving ascattered signal by the scatterers in the fluid at said cylindricalnon-diffraction-grating transducer; determining the changing phase ofthe received scattered signal with respect to the ultrasound beamexcited by the cylindrical diffraction-grating transducer; anddetermining the velocity of the fluid based on the changing phase;whereby said cylindrical diffraction-grating transducer and saidcylindrical non-diffraction-grating transducer are rotational symmetricabout the center of the lumen.
 9. The method of claim 8, furthercomprising compensating for attenuating fluid by using a voltagecorrecting circuit.
 10. The method of claim 8, wherein said placing saidcylindrical diffraction-grating transducer and said cylindricalnon-diffraction-grating transducer including wrapping inside or outsideof the lumen.
 11. The method of claim 8, wherein said cylindricaldiffraction-grating transducer and said cylindricalnon-diffraction-grating transducer comprising an electroded flexibleplastic film, a piezoplastic transducer layer, and a conductor layer.12. The ultrasonic transducer apparatus of claim 11, wherein saidplastic film is constructed from one of a polyimide and PET material.13. The ultrasonic transducer apparatus of claim 11, wherein saidpiezoplastic transducer layer is constructed from one of a P(VDF-TrFE),Nylon 7, and Nylon 11 material.
 14. The method of claim 8, wherein saidexciting of the ultrasound beam is operating in the MHz frequency range.15. An ultrasonic transducer apparatus for measuring a velocity of afluid flowing through a lumen comprising: a cylindricaldiffraction-grating transducer coupled with a predetermined number ofelectrode elements for exciting a double ultrasound beam at apredetermined angle with respect to the lumen axis; a first cylindricalnon-diffraction-grating transducer and a second cylindricalnon-diffraction-grating transducer each coupled with an electrodeelement for receiving a scattered signal by the fluid, wherein saidfirst and second cylindrical non-diffraction-grating transducers areeach placed adjacent to and on opposite sides of said cylindricaldiffraction-grating transducer; a means for determining a first changingphase of the scattered signal received at said first cylindricalnon-diffraction-grating transducer with respect to the ultrasound beamexcited by said cylindrical diffraction-grating transducer; a means fordetermining a second changing phase of the scattered signal received atthe second cylindrical non-diffraction-grating transducer with respectto the ultrasound beams excited by the cylindrical diffraction-gratingtransducer; a means for determining the velocity of the fluid based onsaid first changing phase and said second changing phase; whereby saidcylindrical diffraction-grating transducer and said first and secondcylindrical non-diffraction-grating transducers are rotational symmetricabout the center of the lumen.
 16. The ultrasonic transducer apparatusof claim 15, further comprising a voltage correcting circuit operativeto compensate for attenuating fluid.
 17. The ultrasonic transducerapparatus of claim 15, wherein said cylindrical diffraction-gratingtransducer and said first and second cylindrical non-diffraction-gratingtransducers comprising an electroded flexible plastic film, apiezoplastic transducer layer, and a conductor layer.
 18. The ultrasonictransducer apparatus of claim 17, wherein said plastic film isconstructed from one of a polyimide, and PET material.
 19. Theultrasonic transducer apparatus of claim 17, wherein said piezoplastictransducer layer is constructed from one of a P(VDF-TrFE), Nylon 7, andNylon 11 material.
 20. An ultrasonic transducer apparatus for measuringa velocity of a scattering fluid flowing through a lumen comprising: afirst cylindrical diffraction-grating transducer coupled with apredetermined number of electrode elements for exciting a doubleultrasound beam at a first frequency at a predetermined angle withrespect to the lumen axis; a second cylindrical diffraction-gratingtransducer coupled with a predetermined number of electrode elements forexciting a double ultrasound beams at a second frequency at apredetermined angle with respect to the lumen; a cylindricalnon-diffraction-grating transducer coupled with an electrode element forreceiving a scattered signal by the scattering fluid, wherein said firstand second cylindrical diffraction-grating transducers are each placedadjacent to and on opposite sides of said cylindricalnon-diffraction-grating transducer; a means for determining a firstchanging phase of the scattered signal received at the cylindricalnon-diffraction-grating transducer corresponding to the doubleultrasound beam excited by said first cylindrical diffraction-gratingtransducer; a means for determining a second changing phase of thescattered signal received at the cylindrical non-diffraction-gratingtransducer corresponding to the double ultrasound beam excited by saidsecond cylindrical diffraction-grating transducer; a means fordetermining the velocity of the fluid based on said first changing phaseand said second changing phase; whereby said first and secondcylindrical diffraction-grating transducers and said cylindricalnon-diffraction-grating transducer are rotational symmetric about thecenter of the lumen and said first and second frequencies are different.21. The ultrasonic transducer apparatus of claim 20, further comprisinga voltage correcting circuit operative to compensate for attenuatingfluid.
 22. The ultrasonic transducer apparatus of claim 20, wherein saidfirst and second cylindrical diffraction-grating transducers and saidcylindrical non-diffraction-grating transducers comprising a flexibleplastic film, a piezoplastic transducer layer, and a conductor layer.23. The ultrasonic transducer apparatus of claim 22, wherein saidplastic film is constructed from one of a polyimide and PET material.24. The ultrasonic transducer apparatus of claim 22, wherein saidtransducer layer is constructed from one of a P(VDF-TrFE), Nylon 7, andNylon 11 material.